The present invention relates to electrographic printers of the type wherein a printhead array generates charge carriers and directs them at a recording or imaging member by the selective activation of crossed (e.g., row and column) electrodes. It is particularly directed to such printers wherein the printhead array is a wide array that is substantially the width of an image line, and wherein one set of electrodes is activated with a voltage to function as a source of charge carriers which are extracted by activation of electrodes of the other set.
Printheads of this type are described in U.S Pat. No. 4,160,257, U.S. Pat. No. 4,628,227, and others. In the printheads described more particularly in the aforesaid patents, a set of high voltage electrodes are activated with an RF frequency signal of several thousand volts amplitude to create a localized corona or glow discharge region, and one or more control electrodes located at or near the discharge region are activated in a synchronized manner with lesser control voltages to gate positive or negative charge carriers from the region, thus depositing latent image charge dots on a dielectric imaging member moving past the printhead.
In printing devices using this type of printhead, the RF driven corona generation lines extend along the width of the printhead, spanning many of the control electrodes, which cross them at an angle. One commercial embodiment, by way of example, has twenty parallel RF lines, which are crossed by one hundred twenty eight oblique control electrodes, known as finger electrodes. During the time when one RF line is activated, by a burst of about ten cycles of a one to three MHz drive signal at approximately 2700 volts peak to peak amplitude, those finger electrodes which cross that RF line at the desired dot locations are activated to deposit charge dots.
In the conventional drive circuitry for such systems, the RF drive lines are actuated in a fixed sequence independently of the image being printed. During any given RF line actuation, the number of finger electrodes which are actuated varies in accordance with the Pattern being printed. When the printer is used to print text, the spaces between lines, amounting to approximately fifty percent of the page, remain blank and no finger electrodes are actuated as the corresponding region of the latent imaging belt or drum passes the RF electrodes. In this case, even without any finger actuations, the RF lines continue to be actuated.
The repetitive actuation of the printhead structure with the RF signal, which is applied between electrodes separated by a thin dielectric layer, ultimately leads to localized breakdown or failure of the dielectric material. It also forms atmospheric reaction products and residues which can erode the physical structures of, or impair the operation of, the outermost electrodes and the electrode cavity structures.
In addition, the repetitive actuation of the RF electrodes is the major factor affecting the lifetime of the circuitry for driving the RF electrodes.
Accordingly, it would be desirable to reduce the degradation caused by repetitive RF line actuation.